1. Field of the Invention
The present invention relates to a driving method suitable for a display device using a display medium such as liquid crystal in which pixels are arranged in a matrix. Also, the present invention relates to a display device which conducts display by using the driving method. In particular, the present invention relates to an active matrix liquid crystal panel (liquid crystal panel) of the direct vision type.
2. Description of the Related Art
In recent years, a technique by which a semiconductor device in which a semiconductor thin film is formed on an insulating substrate, such as a thin film transistor (TFT), has been rapidly developed. The reason is that the liquid crystal panel (representatively, an active matrix liquid crystal panel) is increasingly demanded.
The active matrix liquid crystal panel is so designed that electric charges going into or out of pixels of several hundred thousands to several millions which are arranged in a matrix are controlled by pixel switching elements, thus displaying an image.
In the present specification, the pixel is directed to an element which is mainly made up of a switching element, a pixel electrode connected to the switching element and a counter electrode so disposed as to be opposed to the pixel electrode through the liquid crystal.
Hereinafter, a description will be given in brief of a representative example of the display operation of the active matrix liquid crystal panel with reference to FIGS. 19A and 19B.
A source signal line driving circuit 103 and source signal lines S1 to S6 are connected to each other. Similarly, a gate signal line driving circuit 104 and gate signal lines G1 to G5 are connected to each other. A plurality of pixels 106 are disposed in a pixel portion 105 surrounded by the source signal lines S1 to S6 and the gate signal lines G1 to G5. Each of the pixels 106 is equipped with a switching element 101 and a pixel electrode 102. The numbers of source signal lines and gate signal lines are not limited to those values (FIG. 19A). FIG. 19B is a diagram showing the positions of the plural elements 106 disposed in the pixel portion 105 (display pattern).
A video signal is supplied to the source signal line S1 in response to a signal from a shift register circuit or the like (not shown) within the source signal line driving circuit 103. Also, a select signal is supplied to the gate signal line G1 from the gate signal line driving circuit 104, to thereby turn on the switching element 101 of a pixel (1,1) in a portion where the gate signal line G1 and the source signal line S1 cross each other. Then, the video signal is supplied to the pixel electrode of the pixel (1,1) from the source signal line S1. The liquid crystal is driven by the potential of the video signal thus supplied, and the amount of transmitted light is controlled, to thereby display a part of the image on the pixel (1,1) (an image corresponding to the pixel (1,1)).
Subsequently, while a state in which the image is displayed on the pixel (1,1) is maintained by storage capacitors (not shown) or the like, a video signal is supplied into the source signal line S2 in response to a signal from a shift register circuit or the like (not shown) within the source signal line driving circuit 103 on the subsequent instant. In a state where the select signal is continued to be supplied to the gate signal line G1 from the gate signal line driving circuit 104, the switching element 101 of a pixel (1,2) in a portion where the gate signal line G1 and the source signal line S2 cross each other is turned on. Then, the potential of the video signal is applied to the pixel electrode of the pixel (1,2) from the source signal line S2. The liquid crystal is driven by the potential of the video signal thus supplied, and the amount of transmitted light is controlled, to thereby display a part of the image on the pixel (1,2), the same way as the pixel (1,1) (an image corresponding to the pixel (1,2)).
The above display operation is sequentially conducted, and a part of the image is displayed on the pixels (1,1), (1,2), (1,3), (1,4), (1,5) and (1,6) which are connected to the gate signal line G1 in sequence. During this operation, the select signal is continued to be supplied to the gate signal line G1.
Upon supply of the video signal to all of the pixels connected to the gate signal line G1, the select signal is stopped from being supplied to the gate signal line G1, and subsequently the select signal is supplied to only the gate signal line G2. Then, a part of the image is displayed on the pixels (2,1), (2,2), (2,3), (2,4), (2,5) and (2,6) which are connected to the gate signal line G2 in sequence. During this operation, the select signal is continued to be supplied to the gate signal line G2. All of the gate signal lines are subjected to the above display operation, to thereby display one screen (frame) on a display area. This period is called “one frame period” (FIG. 19B).
Until a part of the image is displayed on a pixel (4,6) to which the video signal is finally supplied, all of other pixels retain a state where the image is displayed by the storage capacitors (not shown) or the like.
Those display operation is sequentially repeated, to thereby display the image on the pixel portion 105.
As usual, in the liquid crystal panel using TFTs or the like as the switching elements, in order to prevent a liquid crystal material from being deteriorated, the polarity of the potential of the signal supplied to each of the pixels is inverted (alternating current inverse driving) on the basis of a common potential.
As one inverse driving method, there has been proposed a source line inverse driving method. FIG. 20A shows the polarity pattern of the pixels in the source line inverse driving operation. The polarity pattern shown in FIG. 20 corresponds to the display pattern shown in FIG. 19B.
In FIGS. 20, 22 and 23 showing the polarity pattern, if the potential of the video signal which is supplied to the pixels is positive on the basis of the common potential, the polarity is indicated by “+”, whereas if it is negative, the polarity is indicated by “−”.
In addition, as a scanning method, there has been proposed an interlace scanning method in which every two gate signal lines of one screen (one frame) are jumped over to conduct the scanning operation twice (two fields), and a non-interlace scanning method in which the scanning operation is conducted in order without jumping over the gate signal lines. In this specification, an example in which the non-interlace scanning method is employed will be mainly described.
As shown in FIG. 20A, the feature of the source line inverse driving operation resides in that, in an arbitrary one-frame period, the video signals of the same polarity are supplied to all of the pixels which are connected to the same source signal line, and the video signals opposite to each other in the polarity are supplied to the pixels connected to the adjacent source signal lines. Then, in a succeeding one-frame period, the video signals opposite in polarity to that of the polarity pattern (1) displayed in a one-frame period immediately before the current one-frame period are supplied to the respective pixels, to thereby display a polarity pattern (2).
Also, as another inverse driving method, there has been proposed a gate line inverse driving method. The polarity pattern of the gate line inverse driving method is shown in FIG. 20B.
As shown in FIG. 20B, in an arbitrary one-frame period, the video signals of the same polarity are supplied to all of the pixels which are connected to the same gate signal line, and the video signals opposite to each other in the polarity are supplied to the pixels connected to the adjacent gate signal lines. Then, in a succeeding one-frame period, the video signals opposite in polarity to that of the polarity pattern (3) displayed in a one-frame period immediately before the current one-frame period are supplied to the respective pixels, to thereby display a polarity pattern (4).
In other words, as in the above conventional source line inverse driving method, the gate line inverse driving method is a driving method in which two sorts of polarity patterns (polarity pattern (3) and polarity pattern (4)) are repeatedly displayed.
In recent years, the liquid crystal panel has been demanded to be made thin, light in weight as well as high in precision, high in image quality and high in luminance.
In order to make the liquid crystal panel thin and light in weight, it is necessary to make the substrate size of the liquid crystal panel small. In order to make the substrate size small while an image quality is not deteriorated, the pixel pitch must be unavoidably shortened to reduce the area of the pixel portion.
FIG. 21 shows an enlarged view of the pixels of the liquid crystal panel. As shown in FIG. 21, there are disposed a source signal line 12a, a gate signal line 12b, a pixel TFT (switching element) 15 having a semiconductor layer 13 and a gate electrode 14 which is formed of a part of the gate signal line 12b, and a pixel electrode 16. Then, a black matrix 17 is disposed on the source signal line 12a, the gate signal line 12b and the pixel TFT 15 so as to cover a region which is not required to transmit a visible light therethrough. The black matrix (BM) is directed to wirings (the source signal line 12a, the gate signal line 12b) which are not required to transmit the visible light, or a light shield film disposed about the pixel TFT 15 and so on.
The pixel pitch L is directed to a shorter distance of a distance between two source signal lines 12a opposed to each other through the pixel 11 or a distance between two gate signal lines 12b opposed to each other. If both of those distances are equal to each other, the distances are regarded as the pixel pitch L.
As the pixel pitch is shorter, a distance between two pixel electrodes 16 provided in the adjacent pixels is also shortened more. For that reason, if the source line inverse driving operation and the gate line inverse driving operation are conducted, stripes called “discrination line” occur between the adjacent pixels to which signals of the inverse polarities are applied, whereby the brightness of the entire display screen tends to be reduced.
The disorder (discrination) of the orientation state of the liquid crystal is caused by a potential difference occurs between a pixel to which a video signal of the positive polarity is supplied and a pixel to which a video signal of the negative polarity is supplied, and a display failure (optical loss in case of normally white, light leakage in case of normally black) caused by the disorder of the orientation state is called “discrination line” in the present specification.
The potential difference occurring between the adjacent pixels is generated by a line of electric force shown in FIG. 22A. FIG. 22A shows a top view of a state of the line of electric force occurring between two pixel electrodes A and B with respect to an effective electric field (positive or negative) perpendicular to a paper surface which is applied to the pixel electrodes A and B provided two adjacent pixels, and FIG. 22B shows a cross-sectional view of FIG. 22A. For convenience, FIG. 22A shows only the line of electric force occurring laterally between the pixel electrodes A and B, and FIG. 22B shows a state view of the line of electric force immediately before the liquid crystal molecules the orientation of which is controlled to a perpendicular direction react on application of the electric field.
The discrination pattern corresponding to FIG. 20A is shown in FIG. 22C. In FIG. 22C, the discrination line is formed at a given position, and the discrination pattern (1) and the discrination pattern (2) are substantially identical with each other although the polarity of the video signal which is supplied to the pixel is different therebetween. The discrination line shown in FIG. 22C is found even in the gate line inverse driving method. In case of the gate line inverse driving method, the discrination line appears between the respective pixels in parallel with the direction of the gate signal lines.
In addition, as another conversion driving method which is not shown, there has been proposed a method (dot inverse driving method) in which the polarity of the video signal which is supplied to the pixel is inverted among all of the adjacent pixels. In the dot inverse driving method, the adjacent pixels are different in polarity, thereby greatly influencing the potential difference occurring between the adjacent pixels. In particular, the discrination more greatly influences display as the pixel pitch becomes shorter.
As the pixel pitches are shorter, the distances between the adjacent pixel electrodes becomes more shortened. The discrination is particularly remarkable if the distance is 20 μm or less.
Under the above circumstances, there has been proposed that the source line inverse driving method, the gate line inverse driving method and the dot inverse driving method are replaced by a frame inverse driving method in which the polarities of the video signals which are supplied to all the pixels for each one-frame period are inverted, to thereby suppress the discrination.
FIG. 23 shows the polarity pattern of the respective pixels in the frame inverse driving method. The feature of the frame inverse driving method resides in that the video signals of the same polarity are supplied to all of the pixels within an arbitrary one-frame period (polarity pattern (5)), and in a succeeding one-frame period, the polarities of the video signals which are supplied to all of the pixels are inverted to conduct display (polarity pattern (6)). In other words, if attention is paid to only the polarity pattern, the frame inverse driving method is a driving method in which two sorts of polarity patterns (the polarity pattern (5) and the polarity pattern (6)) are repeatedly displayed. For that reason, in the same frame period, the polarities of the video signals which are supplied to the adjacent pixels are identical with each other, and the discrination is suppressed from occurring.
However, the frame inverse driving method suffers from such a problem that because the brightness of a screen is slightly different between display where the polarity of the image signal is positive and display where the polarity of the image signal is negative, an observer observes the display with flicker. The cause for generating the flicker will be described below in detail.
FIG. 24 shows a timing chart of a video signal supplied to the source signal lines S1 to Sn, a select signal supplied to the gate signal line G1, and the potential of a pixel electrode provided in a pixel (1,1). It is assumed that period during which the select signal is supplied to the gate signal line G1 is a one-line period, and a period of since the select signals are supplied to all of the gate signal lines until one image is displayed is a one-frame period.
Upon supply the video signal and the select signal to the source signal line S1 and the gate signal line G1, respectively, the potential of the video signal positive in polarity which is selected by the select signal is applied to the pixel (1,1) disposed in a portion where the source signal line S1 and the gate signal line G1 cross each other. Then, the potential is ideally retained by a storage capacitor or the like for a one-frame period.
However, in fact, upon completion of the one-line period, the select signal is not supplied to the gate signal line G1, and the potential of the gate signal line G1 is changed. Simultaneously, the potential of the pixel electrode is also changed. The gate signal line is connected to the gate electrode of a pixel TFT which is the switching element of the pixel. Then, the source signal line is connected to one of the source region or drain region of the pixel TFT, and the pixel electrode is connected to the other region which is not connected to the source signal line. A small capacitor is formed between the gate electrode and the pixel electrode, and as the potential of the gate signal line G1 changes, the potential of the pixel electrode also changes by ΔV. In this case, the potential of the pixel electrode changes in a negative direction. In the timing chart shown in FIG. 24, the actual potential of the pixel electrode is denoted by a solid line, and the potential of the pixel electrode assuming that no capacitor is formed between the gate electrode and the pixel electrode is denoted by a dotted line.
Subsequently, in a second frame period, the video signal negative in polarity which is opposite to that in the first frame period is supplied to the pixel electrode provided in the pixel (1,1). Upon completion of the one-line period during the second frame period, the select signal is not supplied to the gate signal line G1, and the potential of the gate signal line G1 changes, with which the potential of the pixel electrode also changes by ΔV in the negative direction.
In other words, assuming that the potential difference between the potential of the pixel electrode and the common potential after completion of the one-line period during the first frame period is V1, and the potential difference between the potential of the pixel electrode and the common potential after completion of the one-line period during the second frame period is V2, a difference of 2×ΔV is produced between the potential difference V1 and the potential difference V2. As a result, the brightness of the screen is different between the first frame period and the second frame period.
Similarly, in case of the source line inverse driving method, the gate line inverse driving method and the dot inverse driving method, the brightness of the screen is different between the pixel to which the video signal negative in polarity is supplied and the pixel to which the video signal positive in polarity is supplied. However, because the pixels different in brightness are adjacent to each other, it is difficult for the observer to recognize the different brightness. On the contrary, in case of the frame inverse driving method, the polarities of the adjacent pixels are completely identical with each other, and the polarities are inverted during the one-frame period which is in a frequency band (about 30 Hz) which can be recognized by human's eyes. As a result, that the display when the polarity of the video signal is positive and the display when the polarity of the video signal is negative are slightly different from each other is recognized as flicker by the observer. In particular, the flicker is remarkably recognized in an intermediate tone display.
As described above, in the source line inverse driving method and the gate line inverse driving method, as exemplified by FIGS. 20A and 20B, the polarity pattern (1) and the polarity pattern (2) are repeatedly displayed, and the discrination lines are continuously formed at a given position between the adjacent pixels different in polarity. As a result, the brightness of the screen is reduced. In addition, the same is applied to the dot inverse driving method.
Also, in the frame inverse driving method, although no discrination is generated, flicker occurs.